Some people theorize that a very heavy dew was the irrigation project God instilled in the garden.
Genesis 1:6-8 New King James Version (NKJV)
6 Then God said, “Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.” 7 Thus God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament; and it was so. 8 And God called the firmament Heaven. So the evening and the morning were the second day
In the outer atmosphere there may have been a blanket of water vapor which supplied evaporation and condensation hence the heavy dew or fog.
If there had been a 'vapour canopy' what do the you think the relative humidity would been and what affect would that have had on evaporation?
Im not saying I agree with the "canopy theory". The dew tho I do . Im not sure how it worked but we still get dew today without a canopy.
Here is what CMI says about it:
Though he says that there may well have been a canopy of some sort with water vapor, any attempt to put enough water up there for 40 days and nights of rain ends up with a surface temperature which will ‘cook’ all life.